To measure the health of a stream or other body of water, a highly important index called the P:R or production:respiration ratio using dissolved oxygen as the indicator is used. This ratio provides a measure of the gross production of biomass or carbon by a benthic community. The community oxygen respiration and production is a sensitive indicator of stress on the functioning of the benthic stream community.
To assess the effect of a toxicant or contaminant on the quality of fresh water, single species toxicity tests are commonly used on an organism of interest. Such tests measure the rate of synthesis of organic material and the decomposition of organic material and, while beneficial in enhancing prediction, are deficient in that they do not duplicate the complex interacting relationships inherent in an ecological system. This disadvantage can be overcome to some extent by utilizing information obtained from the actual aquatic environment in which the organism lives. Such measurements are obtained using the "open reach" method in determining dissolved oxygen at two or more points in a stream with the difference in oxygen obtained between the points giving an estimate of benthic metabolism. The open reach method, however, is deficient because of the diffusion of gases along the water surface between the various points and due to the variations in the water body.
Such diffusions led to the development of a wide variety of enclosed containers which were placed within the stream or body of water to measure both production and respiration under conditions more closely resembling those of the natural habitat. The major advantage of such containers is that easy measurement of ambient gaseous conditions may be obtained together with eliminating the gas diffusion problem and other uncontrollable variations arising from different testing locations and different testing times as is inherent in the open reach testing method.
The enclosed containers used to date, however, have suffered from various disadvantages. In one container, the use of an enclosed rectangular cavity caused "dead" spots in the corner areas which were not replicated in a natural environment. In another container, the use of a pump mounted externally from the container required a large power supply that is not readily portable, portability being a desirable attribute of such containers. In another container, no flow of water was provided within the container over the substrate. Rather, a mere mixing was done which did not simulate flow conditions such as occur in the natural environment. In yet another container, the flow of water within the container over the substrate was not uniform but, rather, was highly uneven. This, again, did not allow replication of conditions identifiable with the natural state or environment conditions surrounding the benthic substrate.